Biaxially stretched polyester films are superior in mechanical strength, heat resistance, chemical resistance and dimensional stability, so that they have been used in a wide range of applications, such as a base film for magnetic tape, an insulating tape, a photographic film, a tracing film, a food packaging film and the like. On the other hand, since polyester films generally have poor adhesive property, it is a common practice to subject a polyester film to a corona discharge surface treatment and to further form an anchor coating layer, when laminating thereon a layer of magnetic agent, photosensitive agent, mat agent and the like. As such anchor coating agent, a number of materials have been proposed, among which are water soluble or water dispersible polyester or acrylic resin to be used for a film having comparatively higher polarity, which is typically a polyester film (Japanese Patent Unexamined Publication Nos. 54-43017, Japanese Patent Examined Publication No. 49-10243 and U.S. Pat. No. 4,098,952). The above-mentioned polyester resin is defective in that a film having a layer made therefrom is susceptible to blocking when rolled, and the above-mentioned acrylic resin is defective in that it sometimes shows insufficient adhesion to a layer to be formed on the anchor coating layer. In an attempt to overcome these drawbacks, it is proposed to use the above-mentioned polyester resin and acrylic resin in mixture (Japanese Patent Unexamined Publication No. 58-124651), though such method does not always satisfactorily compensate for the drawbacks.
It has been also proposed to use various modified polyesters typically represented by one modified by grafting. For example, Japanese Patent Unexamined Publication Nos. 2-3307, 2-171243 and 2-310048 teach that a resin obtained by grafting a compound having an unsaturated bond to a water soluble or water dispersible polyester resin having a hydrophilic group is suitable as an anchor coating agent for a polyester film. Nevertheless, such a graft-modified resin, which has been made to have a hydrophilic group in advance in the polyester resin by, for example, copolymerization, is devoid of high adhesive property or water resistance.
In addition, Japanese Patent Unexamined Publication No. 3-273015 and U.S. Pat. No. 5,015,169 disclose that a graft-modified polyester resin is useful as an anchor coating agent for a polyester film. While such resin shows improved adhesive property in a dry state, it has poor cohesion, such that adhesive property under humidity becomes less and the film increasingly suffers from peeling off of a layer, development of flaws and the like, as the number of process increases.
It is therefore an object of the present invention to resolve the above-mentioned problems and provide a laminated polyester film having superior adhesive property, water resistance and resistance to blocking.